CN1981376A - Buried-contact solar cells with self-doping contacts - Google Patents

Abstract

A buried-contact solar cell, in-process buried-contact solar cell components and methods for making buried contact solar cells wherein a self-doping contact material is placed in a plurality of buried-contact surface grooves. By combining groove doping and metallization steps, the resulting solar cell is simpler and more economical to manufacture.

Description

Buried contact solar cells with self-doping contacts

Cross References to Related Applications

This application claims priority to U.S. Provisional Patent Application Serial No. 60/542,454, filed February 5, 2004, entitled "Process for Fabrication of Buried-Contact Cells Using Self-Doping Contacts," and filed on February 5, 2004 Priority to U.S. Provisional Patent Application Serial No. 60/542,390 filed on the 5th, titled "Fabrication of Back-Contact Silicon Solar Cells". This application relates to Peter Hacke and James M. Gee's concurrently filed U.S. utility patent application Attorney Docket No. 31474-1005-UT, entitled "Contact Fabrication of EmitterWrap-Through Back Contact Silicon Solar Cells", and It involves the US utility patent application Attorney Docket No. 31474-1006-UT, also filed together with James M. Gee and Peter Hacke, and the name of the application is "Back-Contact Solar Cells and Methods for Fabrication". The specifications of all said applications are hereby incorporated by reference.

Background of the invention

Field of invention (technical field):

The present invention relates to photovoltaic solar cells for generating electrical power directly from light, whether the light is natural sunlight or artificial light, and more particularly to solar cells and methods of making solar cells, said solar cells comprising into the contacts on the front and/or back surface of the battery.

Background technique:

Note that the following discussion draws on a number of publications and references. The discussion of such publications herein is given for a fuller background of the scientific rationale and it should not be considered an admission that such publications are prior art for purposes of determining patentability.

In a typical "buried contact" silicon solar cell, the current-collecting grid is recessed into a groove in the front surface. By minimizing the surface area occupied by the gate contact (ie, gate blanking), a larger area available for current collection is obtained. However, even with a small surface contact area, the series resistance loss does not increase because the contact area increases relative to the contact depth and the conductor has a larger cross-sectional area. Other advantages of buried contact cells include extensive diffusion only within the recesses of the buried contacts (reducing contact resistance and losses due to recombination of electrons and holes at contact), and contact metallization being selectively deposited only in the groove. Buried contact cells and methods of making such cells are described, for example, in US Patent Nos. 4,726,850 and 4,748,130. High-efficiency large-area buried-contact cells have been demonstrated on both monocrystalline and polycrystalline silicon substrates.

The representative process steps for fabricating buried contact cells are as follows:

1. Alkaline etching

2. A small amount of phosphorus diffusion (60 to 100Ω/sq)

3.HF etching

4. Deposit silicon nitride on the front surface or both surfaces

5. Laser scribing and etching grooves on the front surface

6. A large amount of phosphorus diffuses in the groove (<20Ω/sq)

7. Deposition of aluminum on the back surface

8. Alloying the aluminum alloy through the back dielectric layer

9.HF etching

10. Deposit Ni thin layer in the groove by chemical immersion plating

11. Sintered Ni layer

12. Deposition of Cu in grooves by chemical immersion plating

As shown in Figure 1A, according to prior art methods, a buried contact solar cell fabricated from a silicon substrate 10 comprises a small amount of phosphorous diffusion 12 above the illuminated surface, a dielectric layer deposited or thermally grown on the front surface 18 and the groove 20 subsequently adopted. As shown in FIG. 1B , after the groove 20 is prepared, a large amount of phosphorus is diffused 30 , such as by preferably using phosphorus oxychloride (POCl ₃ ), phosphine (PH ₃ ), phosphorus tribromide (PBr ₃ ) or other gaseous phosphorus Gas diffusion of the precursor takes place, and in a subsequent step the aluminum layer is coated and alloyed to form an aluminum alloyed connection 50 on the back surface of the cell. Subsequently, the heavily diffused recess 20 is filled with a metal, such as a thin layer 42 of electrolessly deposited Ni, which is sintered, and then a layer 40 of Cu is electrolessly deposited. As shown in FIG. 1C , the final structure results in a groove with an inner surface 30 that is heavily doped (e.g., with extensive phosphorous diffusion), thereby reducing contact resistance and contact recombination, and resulting in metal gates or contacts 40, 42 . Alternatively, a silver (Ag) metal paste can be applied to the heavily doped grooves, followed by firing, as disclosed in US Patent No. 4,748,130.

State-of-the-art buried contact cells have a number of advantages, including a small amount of phosphorus diffusion above the emitting surface for high collection efficiency, a large amount of phosphorus diffusion in the groove for low contact resistance and low contact recombination, and a large amount of phosphorus diffusion to the Mass phosphorus diffusion of grooves and automatic adjustment of electroless metallization stratification. There are some simple variations in the prior art approach, such as cutting the grooves using a dicing or diamond saw instead of using a laser scribe (although laser patterns can provide finer line geometries). The primary disadvantage of the prior art process steps is that the process is relatively complex, time-consuming and expensive. It is advantageous to omit some process steps to obtain the same or improved device structure, eg omit the second diffusion of gaseous phosphorus into the groove. In addition, the elimination of electroless immersion plating is also advantageous because electroless immersion plating involves the use of hazardous chemicals that need to be strictly controlled in the wastewater treatment process.

The use of self-doping metal contacts on the surface of solar cells has been described by Meier et al. in US Patent 6,180,869. See also Daniel L. Meier et al., "Self-doping contacts to silicon using silver coated with a dopant source," 28 ^th IEEE Photovoltaic Specialists Conference, p. 69 (2000). To prepare doped silicon layers and at the same time provide contacts, a paste of Ag:dopants can be placed directly on the silicon surface or fired through the silicon nitride layer, although in the latter case the paste Must contain components that dissolve nitrides (see M.Hilali, et al., "Optimization of self-doping Ag paste firing to achieve high fill factors onscreen-printed silicon solar cells with a 100 ohm/sq. emitter," 29 ^th IEEE Photovoltaic Specialists Conf., New Orleans, LA, May 2002). However, this self-doping metal contact method is only applied to the surface and suffers from high shadowing loss due to the self-doping paste diffusing out on the cell surface.

Summary of the invention

The present invention is a method of preparing a solar cell, the method comprising the steps of: scribing at least one groove in a battery substrate, placing a self-doping contact material in the groove, and heating the self-doping contact Dot material step, whereby doping and metallization of the grooves are performed simultaneously. The self-doping contact material preferably comprises a silicon dopant, preferably a phosphorus-containing silicon dopant. The self-doping contact material preferably comprises silver, and is preferably a paste comprising silver particles and phosphorous. The cell substrate preferably comprises a p-type silicon substrate and the self-doping contact material preferably comprises silver and n-type silicon dopants. Alternatively, the cell substrate comprises an n-type silicon substrate and the self-doping contact material comprises silver and p-type silicon dopants.

The heating step preferably also includes alloying a back contact on the back surface of the cell, and the back contact preferably comprises aluminum. In the method, scribing at least one groove preferably includes scribing a plurality of grooves, wherein the depth of the groove is a multiple of the width of the groove.

The present invention is also a solar cell prepared by the aforementioned method.

The present invention is also a substrate for the production of solar cells, said substrate comprising a planar semiconductor substrate having a front surface and a back surface, at least one groove scribed in said semiconductor substrate surface, located in the groove The self-doping contact material within. The self-doping contact material preferably comprises a paste, and preferably comprises silver. The semiconductor substrate preferably comprises crystalline silicon, and the self-doping contact material comprises a silicon dopant, preferably containing phosphorous. The semiconductor substrate preferably comprises a p-type silicon substrate and the self-doping contact material preferably comprises silver and n-type silicon dopants. The semiconductor substrate preferably further comprises a phosphorous diffusion layer on the surface of the groove. The substrate of the present invention preferably further comprises an aluminum layer on the surface opposite to the grooved surface.

The invention is also a solar cell comprising at least one recess and a contact substantially within the recess, the contact comprising a dopant. The contacts preferably comprise silver and the dopant preferably comprises a silicon dopant, preferably a phosphorus-containing silicon dopant.

One of the main objects of the present invention is to simplify the process for preparing buried contact solar cells.

Another object of the present invention is to provide simultaneous doping and formation of metal contacts within buried contact solar cells.

A major advantage of the present invention is that it provides less expensive solar cells with a minimal amount of grid shadowing.

Another advantage of the present invention is that it is used to eliminate multiple steps in the preparation of buried contact solar cells, including the elimination of bulk phosphorus diffusion steps as well as for electroless plating or other introduction of one or more metals to form Separate step for embedding contacts.

Other purposes, advantages, new features and further scope of applicability of the present invention will be described in part in the following detailed description in conjunction with the accompanying drawings, and part of the content will become apparent to those skilled in the art after verifying the following content Or it can be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

Brief description of several views of the accompanying drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the attached picture:

FIG. 1A depicts a cross-section of a prior art silicon substrate in a grooved fabrication method;

FIG. 1B depicts a prior art silicon substrate after extensive diffusion of phosphorous into the grooves;

Figure 1C depicts a prior art silicon substrate forming a buried contact solar cell with the addition of an electroless metallization layer;

Figure 2A depicts a cross-section of a silicon substrate with a small amount of phosphorous diffused and grooved;

Figure 2B depicts the placement of self-doping metal contact material within a groove according to the present invention; and

Figure 2C depicts a cross-section of a buried contact solar cell after annealing according to the present invention.

Detailed description of the invention

By using self-doping contacts, the present invention allows the elimination of the chemical immersion plating step as well as provides a simple method for making buried contact solar cell structures where the buried contact structures can be in both the front cell surface and the back cell surface On one or both, the self-doping contacts include, but are not limited to, a paste of Ag:dopants.

The silicon substrate is typically polycrystalline or polycrystalline silicon, but other types of silicon substrates can be used, including but not limited to single crystal, tricrystalline, and thin crystalline silicon films on glass or other substrates. Typically silicon is a p-type semiconductor substrate. However, as described below, the present invention can also use n-type semiconductor substrates.

A preferred embodiment of the present invention provides a method for fabricating buried contact cell structures which eliminates a number of diffusion steps and replaces the electroless plating step (two plating steps) with a single printing and firing step. and primary sintering). This method is primarily enabled by filling the grooves with self-doping contact material. Self-doping contact materials contain both: elemental metals or alloys, and silicon dopants that dope the silicon surface during contact formation. To make an n-type layer, the silicon dopant is an n-type dopant such as phosphorus (P), antimony (Sb) or arsenic (As). Alternatively, to prepare a p-type layer, if the substrate is n-type, the dopant is preferably a p-type dopant, such as indium (In), aluminum (Al), boron (B) or gallium ( Ga). Metallic supports are preferably alloyed with silicon at relatively low temperatures and are good conductors. The latter property allows the use of the metal support as a gate line for conducting current from the solar cell. Candidate materials include, for example, Ag, Al, Cu, Sn and Au, where Ag is due to its inert nature (it can be fired in air with minimal oxidation), good conductivity, and compatibility with Si material processing. Capacitive (Ag is not a strong recombination center in Si which can reduce solar cell efficiency) is preferred.

In a preferred embodiment, the self-doping contact material is a paste, preferably a paste containing P-doped Ag particles. The paste is applied to the grooves by any feasible means including screen printing, squeegee application or other printing or deposition means. After coating the Ag:P paste, the solar cells were fired at a temperature above the Ag:Si eutectic temperature (845° C.) to prepare Ag gates with doped contacts. When the temperature exceeds the eutectic temperature, Ag dissolves part of the Si on the interface, and when the temperature decreases, the precipitated silicon is doped with phosphorus. N-type dopants other than P may be used, such as Sb or As, especially but not exclusively in combination with Ag. Similarly, although the use of self-doping pastes is preferred, other forms of Ag may be used, and the use of Ag:dopants may be preferred.

In an alternative embodiment, Ag paste or other Ag paste is applied in the grooves by any feasible means including screen printing, squeegee coating or other printing or deposition including sputtering or evaporation. layer. Subsequently, the Ag layer is coated with a material containing an n-type dopant, such as a layer containing a phosphorus compound. Alternatively, if a p-type layer is required where the substrate is n-type, then the overlying layer can be made to Materials containing p-type dopants such as boron (B), indium (In), gallium (Ga) or aluminum (Al) are included. Alternatively, a dopant-containing layer may be placed between the carrier metal (in this case, Ag) and the Si. After the temperature has been raised above the metal-Si eutectic temperature, the dopant-containing layer is dissolved in the metal-Si liquidus by this method.

Pastes can be prepared by combining Ag in particle form with dopants in liquid phase, thereby producing self-doping, screen-printable pastes. To prepare a screen-printable paste, the paste formulation may also contain binders, solvents, etc. that are known and used in the art. It is also possible and conceivable to use sintered paste formulations, such as pastes containing glass frit, especially where penetration through the silicon nitride layer is required.

Self-doping Ag metallization uses only one step—printing or otherwise placing the self-doping contact material in the grooves instead of the extensive phosphorous diffusion step and three metal plating and sintering steps of existing methods. The resulting process steps thus require fewer steps and are much simpler than those of conventional buried contact cells, while providing all or substantially all of the same high efficiency advantages.

Figure 2A depicts an undoped recess 20 cut into a silicon substrate 10 with a layer of a small amount of phosphorous diffusion 12 pre-coated on the front surface, in accordance with the present invention. The small amount of phosphorous diffusion layer 12 is preferably covered before the grooves are formed by a dielectric layer 18 which acts as an anti-reflection coating for the solar cell and keeps any excess printed on the outer surface of the grooves Metallurgical reactions between the metal and the diffusion layer 12 are minimal. Silicon substrate 10 preferably comprises p-type semiconductor silicon, however, other substrates including n-type silicon and gallium or silicon-gallium substrates of any conductivity type may be used. A small amount of phosphorous diffusion 12 layer is applied by conventional means, preferably including gas phase diffusion using liquid _POCl3 . However, other diffusion sources or methods may be used, including coating a liquid source by conventional methods such as coating, dipping or spin coating, or coating _a solid source such as by heating a solid source material such as _P2O5 to an elevated temperature. In general, however, conventional gas-phase _POCl3 diffusion is preferred.

The grooves 20 can be cut or scored by any method that can produce grooves of the desired size. While laser scribing is preferred, other methods including etching, mechanical milling, etc. may be used. Grooves 20 are longitudinal indentations substantially parallel to one another between opposite edges of the surface.

Note that in the drawings, especially in FIGS. 2A to 2C , the dimensions of the grooves 20 , the thicknesses of the various layers, and other dimensions are not drawn to scale, but are schematically shown for the purpose of explanation and easy identification. Typically, buried contact groove 20 has a depth greater than its width, and in a preferred embodiment, the depth is a multiple of the width. For example, the width of groove 20 may be between about 10 μm and about 50 μm, preferably about 20 μm, and the depth of groove 20 may be between about 20 μm and about 60 μm (depending somewhat on the thickness of substrate 10), preferably about 40 μm . Thus, as shown in FIG. 2A, the groove 20 may not have a linear cross-section, but may have a rounded bottom, sloped sidewalls, or the like. The parallel plurality of grooves 20 are separated by a distance which depends to some extent on battery design considerations. However, any feasible separation may be possible, so adjacent parallel grooves 20 may be separated by a distance (centerline to centerline) of about 1000 μm to about 3500 μm, preferably about 1500 μm to about 2500 μm. The thickness of the dielectric layer 18 (if silicon nitride having a refractive index of about 2 is used) in the completed solar cell is preferably about 80 nm, and the thickness of the diffusion layer 12 is preferably about 200 to 1500 nm.

FIG. 2B depicts groove 20 filled with self-doping contact material 60 . The self-doping material 60 can be any of the above-mentioned ones, and preferably comprises a paste containing Ag and a silicon dopant, and the silicon dopant preferably contains P. However, the self-doping contact material 60 may alternatively be coated with P or other dopant, followed by Ag and dopant (either Ag first then dopant or dopant first then Ag). Dry preparations of Ag particles, or other formulations of self-doping contact materials that can be selectively applied into the grooves simply and inexpensively.

After application of the self-doping contact material 60, the self-doping contact material is alloyed with silicon, preferably by heating or firing at a temperature above the Ag:Si eutectic (845° C.) temperature, to produce Ag gates with self-doped contacts, resulting in the structure shown in Figure 2C. When the temperature exceeds the eutectic temperature, Ag dissolves part of Si on the interface, and when the temperature decreases, the precipitated silicon is doped with phosphorus, thereby obtaining a silicon-doped layer 70 on the inner surface of the groove, in which Ag The contacts 80 occupy the recesses.

An example of the process steps of the present invention using a single phosphorous diffusion step and self-doping paste for the gate is as follows:

1. Alkaline etching

2. A small amount of phosphorus diffusion (60 to 100Ω/sq)

3.HF etching

4. Deposition of silicon nitride on the front surface

5. Scribing grooves on the front surface, preferably using laser scribing grooves

6. Deposit a self-doping paste (eg Ag:P) in the grooves on the front surface

7. Deposition of aluminum on the back surface

8. Furnace annealing to alloy Ag and Al contacts simultaneously

In the preceding steps, alkaline etching is used to clean the surface. Any suitable alkaline etch material can be used, such as hot or warm sodium hydroxide. By way of example, an aqueous sodium hydroxide solution between about 2% and 50% by weight may be used, preferably at a temperature between about 60°C and about 95°C.

A small amount of phosphorus is diffused as described above. After diffusion, an acid etching step is used, such as with aqueous hydrofluoric acid (HF), preferably with 2 to 50% by weight HF acid. Any conventional method may be used, including dipping the wafer in a solution containing HF acid. Oxides originating from small amounts of phosphorous diffusion are preferably but optionally removed with an acid such as HF, as oxides can cause reliability issues, especially for encapsulated optoelectronic modules. After the HF etch, the exposed silicon surfaces are preferably but optionally passivated by depositing a dielectric layer. Silicon nitride (SiN) can be conventionally deposited by plasma enhanced chemical vapor deposition (PECVD) or by low pressure chemical vapor deposition (LPCVD), well known techniques for passivating silicon surfaces in solar cell structures. However, other methods and materials for passivation can be used if desired, such as for example thermally growing _SiO2 layers or depositing other dielectric materials such as _SiO2 , _TiO2 , _Ta2 by various means such as printing, spraying, PECVD, etc. O ₅ et al.

If necessary, grooves are scribed in the front surface after passivation, for example with SiN. Lasers are preferably used, such as Q-switched Nd:YAG lasers. However, mechanical scoring or other means as described above may be used. After optional scribing, a cleaning step may be used, such as a cleaning step using a chemical solution containing sodium hydroxide or potassium hydroxide.

The recess 20 is then filled with a paste of self-doping contact material such as Ag:P. Such filling can be done by means of screen printing, but other means can also be used. Although FIG. 2B shows the self-doping contact material 60 that only fills the groove and only reaches the level of the upper surface of the substrate 10, it is possible and contemplated that the self-doping contact material 60 and the resulting contact 80 (as shown in FIG. 2C shown) may extend beyond the surface and may optionally be hemispherical.

For conventional batteries, any metal back contact can be used. In a preferred embodiment, an Al back contact is used, as described above. Applying Al prior to annealing the Ag:P paste is a particularly preferred embodiment because a single furnace annealing step can be used so that the Ag and The Al forming the back contact is alloyed at the same time. For example, the Al back contact can be applied by e-beam vaporization, sputtering, screen printing or other techniques.

To keep the SiN layer passivated, which can be affected by high metallization temperatures, step 4 can optionally be performed after metallization, plus additional masking and stripping steps. Step 5 can alternatively be performed at the beginning of the process. If the recess 20 is passivated, eg with SiN, the self-doping contact material 60 optionally and preferably comprises glass frit.

In one embodiment, a small amount of phosphorous diffusion may be applied after scribing the grooves 20 . Typically, but not necessarily, such methods, such as passivation using SiN, are performed prior to application of the self-doping contact material 60, so glass frit is preferred. However, this approach has the advantage of providing an additive effect of n-type dopants within the recess 20, i.e., with substantially additional n-type doping through the use of self-doping contact material 60, a small amount Phosphorus diffusion produces partial doping inside the surface of the groove walls. As such, the doping within the groove sidewalls is substantially greater than the surface doping due to the additive effect of successive doping steps.

Although the invention has been described in detail with specific reference to these preferred embodiments, other embodiments are possible with the same results. Variations and modifications of the present invention will be apparent to those skilled in the art and it is intended to cover all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, as well as the corresponding applications, are hereby incorporated by reference.

Claims (23)

1. method that is used to prepare solar cell, described method comprises the steps:

At least one groove of scribing in cell substrate;

The autodoping slider material is placed in the described groove; And

Heating autodoping slider material mixes simultaneously thus and metallizes described groove.

2. according to the process of claim 1 wherein that described autodoping slider material comprises silicon dopant.

3. according to the method for claim 2, wherein said silicon dopant comprises phosphorus.

4. according to the process of claim 1 wherein that described autodoping slider material comprises silver.

5. according to the process of claim 1 wherein that described autodoping slider material is the paste that comprises silver particles and phosphorus.

6. according to the process of claim 1 wherein that described cell substrate comprises p-type silicon substrate and described autodoping slider material comprises silver and n-type silicon dopant.

7. according to the process of claim 1 wherein that described cell substrate comprises n-type silicon substrate and described autodoping slider material comprises silver and p-type silicon dopant.

8. lip-deep back of the body contact alloyization will be carried on the back according to the process of claim 1 wherein that described heating steps also comprises at battery.

9. method is according to Claim 8 wherein carried on the back the contact and is comprised aluminium.

10. according to the process of claim 1 wherein that at least one groove of scribing comprises a plurality of grooves of scribing, the degree of depth of wherein said groove is the multiple of described recess width.

11. a solar cell, it is by the method preparation according to claim 1.

12. a substrate that is used to prepare solar cell, described substrate comprises:

Planar semiconductor substrate with front surface and back of the body surface;

At least one groove of scribing in the surface of described Semiconductor substrate; And

Be placed on the autodoping slider material in the described groove.

13. according to the substrate of claim 12, wherein said autodoping slider material comprises paste.

14. according to the substrate of claim 12, wherein said autodoping slider material comprises silver.

15. according to the substrate of claim 12, wherein said Semiconductor substrate comprises crystalline silicon and described autodoping slider material comprises silicon dopant.

16. according to the substrate of claim 15, wherein said silicon dopant comprises phosphorus.

17. according to the substrate of claim 1, wherein said Semiconductor substrate comprises p-type silicon substrate and described autodoping slider material comprises silver and n-type silicon dopant.

18. according to the substrate of claim 12, wherein said Semiconductor substrate also is included in the phosphorus-diffused layer on the described groove surfaces.

19. according to the substrate of claim 18, described substrate also be included in described groove surfaces opposite surfaces on aluminium lamination.

20. a solar cell, it comprises:

At least one groove; And

Be located substantially on the contact in the described groove, described contact comprises dopant.

21. according to the solar cell of claim 20, wherein said contact comprises silver.

22. according to the solar cell of claim 20, wherein said dopant comprises silicon dopant.

23. according to the solar cell of claim 22, wherein said dopant comprises phosphorus.

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